This invention relates to an imaging spectrometer system and more particularly, to a hyperspectral imaging system operating in an alternating staring/scanning mode for acquiring ground track images from air or space platforms acquiring an optimized pushbroom hyperspectral image data set with photogrammetric reference.
Over the past decade, imaging spectrometry has been emerging as a new remote sensing tool following advances in multispectral imaging technology. Imaging spectrometry, which consists of the acquisition of images in many narrow, contiguous spectral bands, offers considerable advantages over traditional multispectral scanner imaging for remote sensing in terms of high resolution in spatial, spectral, and radiometric domains. Consider the spectral domain for example. The reflected solar radiance, which carries the spectral characteristics of the remotely sensed targets on the earth surface, used to be undersampled (with only a few discrete measurement bands) by multispectral scanners. For instance, a Coastal Zone Colour Scanner (CZCS) in the NIMBUS-7 satellite (1978-1986) has four visible bands at 433, 520, 550 and 670 nm with 20 nm half-width and a Thematic Mapper.TM. of Landsat supplies only six broad spectral bands of images with large gaps over the solar reflected spectral range 0.4 to 2.5 mm. Imaging spectrometers are designed to provide contiguous spectral sampling over the measurement range.
Imaging spectrometry results in hyperspectral image data set (Geotz et al. 1985). By contiguous high-resolution spectral-imaging sampling, the data yielded from the imaging spectrometry approach will increase by over an order of magnitude over the current multispectral scanner data in the same spectral region.
In the prior art an airborne multiband imaging spectrometer is described in U.S. Pat. No. 5,149,959, issued Sep. 22, 1992 to William E. Collins, et al. and assigned to Geophysical & Environmental Research Corporation of New York. The Collins et al. spectrometer is basically a whiskbroom imaging spectrometer. In the most basic whiskbroom approach, a rotating mirror scans the ground crosswise to the path of the airplane. The crosswise data is picked up pixel by pixel. The image is skewed, compared to a photographic image because the crosstrack pixels are not in a line; the later-recorded pixels are further along the ground than the earlier-recorded pixels. However, Collins et al. improve on the basic whiskbroom approach by using a rotating polygon mirror to scan the ground four times per revolution, allowing a factor of four reduction of rotation speed to scan speed. The spectrometer comprises a beam splitter that divides the light into two (or more) beams or so called contiguous bands. Then two (or more) beams are directed to two (or more) diffraction gratings and then to detectors. Each of the detectors is comprised as a line array for each diffraction grating. The detector output is sent to a signal processor, which is chiefly a normal analogue to a digital (A/D) converter. The output data is converted for recording and real time display. However, whiskbroom imaging spectrometers are big and heavy, perhaps 200 kilograms for a typical unit, and mechanically complex. Also, the images are not the same as photographs and there may be skewing.
In U.S. Pat. No. 5,276,321, issued Jan. 4, 1994 to Sheng-Huel Chang et al. and assigned to Geophysical & Environmental Research Corporation, a multi-channel imaging spectrometer for airborne geological surveys and environmental surveys in a moving vehicle similar to U.S. Pat. No. 5,149,959 is described. It comprises an optical scanner employing a rotating polygon scan mirror which achieves a wide lateral field of view in what is known as whiskbroom mode of operation. The energy scanned is directed by a Kennedy scanner to a parabolic mirror and then fed to a plurality of spectrometers. A further improvement provides another mode of operation, known as the "pushbroom" or "staring" mode which utilizes a fixed mirror centered on the nadir and utilizes the motions of the vehicle to scan the scene. In a further improvement, the scene is scanned by the rotating mirror and the image is sequentially applied to a two-dimensional array so as to generate a continuous three-dimensional spectral display. However, this invention employs a complicated rotation polygon as a simple mirror in the pushbroom imaging mode.
A dispersion component-based imaging spectrometer has a one dimensional view. Its dispersed two-dimensional imagery gives no information about a geometric view of the ground track. A band switching video (using a filter wheel or a tuneable filter) imagery sequence is never visually comfortable to a user because of periodic flickering caused by the periodic band changing. In the traditional one-dimensional field of view imaging spectrometers, as described above in U.S. Pat. No. 5,276,021, they are not able to provide directional spectral images.
A wedge image spectrometer (WIS) for collecting imagery is described in a paper by J. G. Thunew and L. M. Woody entitled, "New Sensor Technology for Acquiring Hyperspectral Imagery," First International Airborne Remote Sensing Conference and Exhibition, Strasbourg, France, 11-15, Sep. 1994. The WIS comprises a linear spectral wedge filter with tapered layers. It is a thin-film optical filter that transmits light at a center wavelength that depends on the spatial position of the illumination. If an array of detectors is placed behind the device, each detector now will receive light from the scene at a different center frequency and the array output is the sampled spectrum of the scene. By mating the wedge filter to an area array, the scene information was claimed to vary spatially in one direction and spectrally in the other (Thunew and Woody, 1994). Moving the assembly perpendicular to the spatial dimension builds a two-dimensional spatial image in each of the spectral bands. Another version of the WIS is implemented with a mosaicized filter which comprises two filters with each filter covering less than a spectral octave. However, the actual two dimensional field of view nature of this sort of imaging spectrometer using one area sensor, has associated with it the difficulty of separating the angular effects introduced by the two-dimensional field of view observation. It cannot separate and measure the directional effect. Moreover, it could be hard to focus both the blue light such as at 400 nm, and the near infrared light such as at 1050 nm onto a single focal plane of sensor.
An imaging spectrometer, comprising imaging optics, a spectrometer module and a sensor, is a physical instrument for realizing the imaging spectrometry concept. It can be categorized by the type of the spectrometer adopted. Traditionally a light dispersion spectrometer is employed in an imaging spectrometer design using a grating or prism as the dispersing element. Later, the light-frequency-spatially-selecting spectroscopy concept was developed by the present inventor at University of Dundee, of Dundee Scotland, to construct a spatially linear variable interference filter based imaging spectroscopy instrument (VIFIS) as described in a paper by X. Sun and J. M. Anderson, "A Light-Frequency-Spatially-Selecting Component Based Airborne Pushbroom Imaging Spectrometer for the Water Environment", Proceedings of the First Thematic Conference on Remote Sensing for Marine and Coastal Environments, SPIE Volume 1930, New Orleans, La. p. 1063-1076, 1992. It is simple and compact in structure with advantages in easy deployment, two-dimensional field of view, the same ray geometry to a normal video camera, better picture quality, and application flexibility. However, analog video recording techniques were employed resulting in less than desirable picture quality.
The first two dispersion imaging spectrometers were tested in the early 1980s. They are the airborne imaging spectrometer (AIS) by Jet Propulsion Laboratory (JPL) (Vane, et al. by Moniteq Ltd. of Toronto (Gower et al. 1985). The first airborne single camera variable filter imaging spectrometer was tested in 1991 (see above, Sun and Anderson 1992).
However, the spectral track-recovery-images of VIFIS are three-parameter spectral images dispersed along the correlated wavelength, direction and time-delay parameters. Its pixel spectrum is progressively scanned rather than simultaneously captured and may be subject to the influence of different viewing angles.
In U.S. Pat. No. 5,790,188 issued to Xiuhong Sun on Aug. 4, 1998 and assigned to Flight Landata, Inc. of Lawrence, Massachusetts, a variable interference filter imaging spectrometer (VIFIS) system is described which acquires ground track images from air or space with a two-dimensional field of view and generates imagery from three channels of synchronized video outputs. The synchronized output video from each camera is fed to a control and interface unit where a composite analog signal is formed from the individual output video signals for recording on an analog video recorder. A digital signal is also generated for recording on a computer disk. Control of the shutter speed of each of 3 cameras is provided.
The present invention is a further improvement of the VIFIS system using three camera modules, but having all digital processing and employing a staring/scanning method.